Heimaeyite, Na₃Al(SO₄)₃, a new mineral from the fumaroles on Eldfell volcano, Iceland

Abstract. The new mineral, heimaeyite, was found among the fumarolic encrustations collected on Eldfell volcano, on the island of Heimaey, Iceland. The mineral formed as microscopic rounded aggregates (<50 µm) and rounded grains (<10 µm) mixed with tiny needles of koryakite and a mineral phase with an NaMgAl(SO4)3 composition. It was formed in scoria consisting of anhydrite, hematite, and cristobalite. Other associated minerals are tamarugite, hexahydrite, löweite, and langbeinite. Heimaeyite is orange with a yellowish streak. It is translucent with a vitreous lustre. The calculated density is 2.783 g cm−3. The chemical analysis gives a composition of Na2.93Al0.82Fe0.25S2.99O12.05. The ideal formula is Na3Al(SO4)3 with 10 % to 25 % replacement of Al with Fe. Single-crystal X-ray diffraction analysis shows that it is trigonal and has a space group of R3‾ with unit cell parameters of a=13.4326(9) Å, c=8.9818(7) Å, V=1403.5(2) Å3, and Z=6. The X-ray powder diffraction resembles that of Na3V(SO4)3 (PDF 39-0243), with which it is isostructural. They both belong to the K7Nb(SO4)6 structure type. In the crystal structure, the octahedral coordinations of the cation (in heimaeyite this is Al) are connected by sulfate tetrahedra in a “pin-wheel” arrangement into chains extending along [001]. The chains are interconnected by [NaO6] distorted trigonal prism coordinations. Fe substitutes for Al in the larger of the two symmetry-independent octahedral sites. Synthetic pure Na3Al(SO4)3 was prepared through a solid-state reaction at 550 °C, and its structure was refined by the Rietveld method. It shows a smaller difference in size between the two octahedral sites than the natural sample.